1. Field of the Invention
The invention concerns the ergonomics and safety of head equipment, notably incorporating display devices, intended to be worn by aircraft crew.
The use of so-called visionic display devices mounted directly on the user's head rather than on the instrument panel has many advantages. For example, it is possible to compensate the human vision when ambient light conditions become insufficient by means of light-intensifying night-vision devices. It is also possible to complement the user's natural vision by displaying symbols and synthetic images.
However, these devices incorporating image sources, optical components, mechanical parts, etc. have non-negligible weight and substantially increase the total load supported by the spinal column. This additional load is particularly inconvenient in that its center of gravity does not coincide with that of the head. Aircraft crews are, moreover, subject to very strong mechanical stresses (vibrations, accelerations, ejection thrust, parachute opening, crashes, etc.) which further increase the loads to which the spinal column is subjected. Moreover, the upper spinal column is known to be relative vulnerable to strong accelerations, with a risk of severe injury or even death.
The design of head-mounted visionic systems takes into account this type of risk in an effort to reduce it to an acceptable level by applying general rules and recommendations to limit the equipment mass and better position its center of gravity. FIG. 1 illustrates an example of this type of recommendation: the user's head T and the head equipment C are shown in a profile view; the head is represented by dotted outline and the helmet by a solid outline. The center of gravity CGC of the head equipment is represented by a black dot; the center of gravity CGT of the head is represented by a white dot. The white and black arrows represent the respective weights of the head and the head equipment. The recommendations notably specify that the center of gravity CGC must be located within a zone Z represented by the shaded rectangle whose position is specified relative to the head's center of gravity (anthropometric relationships of body and body segment moments of inertia—Air Force Aerospace Medical Research Laboratory—J. T. McConville, Ints Kaleps—J. Cuzzi—December 80).
Knowledge of the position of the head's center of gravity is therefore important for head equipment designers.
2. Description of the Prior Art
The equipment characteristics are well defined and relatively easy to calculate and measure. CAD (computer-assisted design) tools are able to calculate, on the basis of the design, the provisional weight of the helmet equipment and the position of its center of gravity. On the other hand, it is much more difficult to know precisely the mechanical characteristics of the user's head. So-called “standard” head models are traditionally used. However, these models can lead to false conclusions regarding the anatomic particularities of certain users and incorrect calculation of the head's center of gravity.